The present invention relates to the production of wood pulp by the green liquor semi-chemical pulping method. Green liquor semi-chemical pulping is a two-stage process. It uses green liquor cooking to soften the wood chips and mechanical refining to disintegrate the cooked chips into individual fibers. The present invention utilizes surface active agents having the following general structure: ##STR2## wherein a, b, and c are at least 1 but are such to produce an agent having a molecular weight of 500 to 30,000 with those having a molecular weight of 1,000 to 10,000 being preferred. The surface active agent is added during the green liquor semi-chemical pulping process.
Green liquor typically consists of Na.sub.2 S, Na.sub.2 CO.sub.3, and water. It is used to weaken the intercellular bonding by partial removal of hemicellulose and lignin. The more green liquor that is used, the more hemicellulose and lignin are removed, resulting in less mechanical energy required to refine the cooked chips. This energy savings is counter-balanced because the more hemicellulose and lignin that are removed, the lower the pulp yield. Conversely, the less green liquor used, the more mechanical energy is required and the higher the pulp yield. It is an object of this invention to increase the pulp yield by reducing the green liquor amount without increasing the mechanical energy.
U.S. Pat. No. 3,909,345 discusses and claims the use of surface active agents having the general formula: EQU R[(C.sub.2 H.sub.4 O)n (C.sub.3 H.sub.6 O)m)]y H
as additives to sulfate cooking liquor aids for the purpose of obtaining higher yields of pulp from a given wood chip charge. These agents permit a greater effectiveness of the cooking process relative to chips which prior to that invention were considered rejects and not pulpable.
While there is a degree of similarity between the invention of the '345 patent and that of the present inventors, the similarities cease as regards to the type of surface active agent utilized and the type of pulping process utilized.
The present invention utilizes surface active agents having the general structure: ##STR3## wherein a, b, and c are at least 1 but are such to produce an agent having a molecular weight of 500 to 30,000 with those having a molecular weight of 1,000 to 10,000 being preferred.
The green liquor semi-chemical pulping process differs from the kraft or sulfate pulping process described in the '345 patent. The kraft pulping process is a wholly chemical approach to pulping. Through the use of heat, pressure and chemicals the wood chips are disintegrated into fibers by cooking for about one hour for eventual use as linerboard for example. The typical chemicals utilized are sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S). In contrast, green liquor semi-chemical pulping is a two step process. The wood chips are first softened by the chemical processing involving about a twenty (20) minute cooking time then the softened wood chips are fiberized utilizing mechanical energy for eventual use as corrugated medium in boxboard for example. The typical chemicals comprising the green liquor which is used for softening the wood chips are sodium carbonate (Na.sub.2 CO.sub.3) and sodium sulfide (Na.sub.2 S).